The present invention relates to a four-wheel drive system of a part-time type having a four-wheel drive mode and a two-wheel drive mode, and more specifically to an automatic control system for a part-time four-wheel drive system, which is arranged to change automatically from the four-wheel mode to the two-wheel mode in accordance with a steering angle of the steerable wheels.
A four-wheel drive vehicle is superior in ability of hill climbing and ability of rough road driving. If, however, all four wheels are always driven at the same speed, a four-wheel drive vehicle cannot be turned smoothly. During a turn at a large steering angle of steerable wheels, the front wheel of an inner or outer side must travel along a circle having a large radius and the rear wheel of the same side must travel along a small circle. Because of this difference in turning radius, there arises a large difference between a rotation speed (an average rotation speed, to be exact) of the front wheels and a rotation speed (an average rotation speed) of the rear wheels. As a result, the steering becomes heavy, the tendency to understeer is increased, and the vehicle cannot be turned without abnormal tire friction (called tight corner brake) which tends to brake the vehicle and cause an engine stall.
To overcome these handling and tire friction problems, some four-wheel drive vehicles are provided with a changeover means capable of changing the state of a four-wheel drive system between a two-wheel drive mode and a four-wheel drive mode. This type of four-wheel drive is called a part-time type. One example of the part-time four-wheel drive vehicle are shown in Japanese Patent provisional publication No. 57-84227.
A four-wheel drive vehicle of the part-time type can be turned through a tight corner very smoothly by holding the drive system in the two-wheel drive mode.
However, a part-time type four-wheel drive vehicle is very difficult and troublesome to control especially for the average driver. If a four-wheel drive vehicle of this type is turned at a large steering angle without changing the drive system from the four-wheel drive mode to the two wheel drive mode, abnormally great torques are exerted on ends of front and rear propeller shafts and axle shaft drivingly connected with the front wheels and the rear wheels. Therefore, the steering of the vehicle becomes very heavy, and there arises a danger of damage to the propeller shafts and the axle shafts. Furthermore, the front wheels and the rear wheels slip in the opposite directions during a turn, so that the tendency to understeer is increased and the effect of the tight corner brake is produced. The average driver cannot understand the reason for such abnormal behavior by the vehicle and cannot negotiate a corner smoothly.